The present invention relates to a press-formed product, such as automotive parts, made of steel sheet, and to a method of press-forming such a product. More particularly, the present invention relates to a press-formed product which is nearly free from malformation that otherwise would occur after its release from the forming die, and relates also to a press-forming method for such a press-formed product. (Malformation denotes poor dimensional accuracy resulting from warpage in the wall of formed products or variation in the angle of bent parts of formed products, both due to elastic recovery.)
A car body is usually constructed of a large number of press-formed products produced from steel sheet by press-forming. These press-formed products are produced by draw-bending. Draw-bending is one way of press-forming with a convex and a concave forming die in combination. These dies are designed such that the convex die moves toward the concave die, thereby press-forming a blank which is held on the concave die under pressure exerted by a blank holder. Thus the blank is shaped between the two dies in conformity with their configuration.
The above-mentioned draw-bending has disadvantages as follows. As the convex die enters the concave die, the blank undergoes bending deformation in the vicinity of the blank holder. As the convex die moves further into the concave die, the blank undergoes unbending deformation. Deformation in this way is called bending-unbending deformation. This deformation produces residual stress in the formed product because the blank is compressed and then stretched as the convex die enters the concave die and the stretched blank is subsequently compressed as the convex die moves further into the concave die. Therefore, the outside of the formed product has a residual tensile stress in the direction of pressing and the inside of the formed product has a residual compressive stress in the direction of pressing. Thus there exists a difference of stress in the thickness direction. Similarly, those specific parts of the formed product, which are deformed by the shoulder of the convex die and which undergo bending alone without unbending, have residual stresses in different directions in their surface. This also causes a difference of stress in the thickness direction.
Those parts which have undergone bending deformation or bending-unbending deformation are subject to elastic recovery after press forming and hence they do not have the dimensions and shape as designed. They present difficulties in assembling and joining (usually by spot welding). Otherwise, their assembly is an imperfect car body with dimensions partly or entirely different from the design. This problem has become more serious in recent years as the result of increasing use of automobile steel sheet having higher strength than before for weight reduction and improved safety and as the result of increasing use of aluminum sheet which is light in weight but has a much lower Young""s modulus than steel sheet.
The following three methods have been adopted to address the above-mentioned problem.
(1) A method which consists of applying a tensile force to the side wall in the last stage of forming, thereby causing the formed product to fit with the die. This method is intended to prevent the side wall of the press-formed product from warping.
(2) A method which consists of applying a large compressive force in the thickness direction of the blank in the last stage of forming. This method (called final pressing) is intended to cope with the change in angle of bent parts of the press-formed product.
(3) A method which consists of pushing the punch (convex die) into the center of the blank while firmly holding the periphery of the blank by the blank holder so that the blank will not flow into the concave die. This method is intended to cope with the change in radius of curvature of curved parts of the press-formed product.
The above-mentioned methods, however, have the following disadvantages respectively.
The first method needs a special pressing apparatus or an additional stretching step in the case where an ordinary press is used.
The second method produces only a marginal effect although it is easily practicable with an ordinary pressing apparatus.
The third method, which basically consists of stretching the blank, tends to cause defects (such as rupture) to the blank during forming.
In any way, it is apparent that the conventional methods of eliminating malformation have many problems when they are applied to practical use with an ordinary pressing apparatus.
The present invention has been completed in view of the foregoing. It is an object of the present invention to provide a press-formed product having a good shape easily attainable by an ordinary pressing apparatus. It is another object of the present invention to provide a press-forming method for such a press-formed product.
The first aspect of the present invention is directed to a press-formed product which is characterized by having linear concave portions in the region which has undergone bending or bending-unbending deformation during press-forming, said linear concave portions satisfying the following requirements.
D/Pxe2x89xa70.03xc3x97t/1.2
and
0.02t less than Dxe2x89xa60.5t
where,
P is an interval between the linear concave portions (mm),
D is a depth of the linear concave portions (mm), and
t is a wall thickness of the press-formed product (mm).
The second aspect of the present invention is directed to a method of producing a press-formed product, said method being characterized by using two forming dies in combination, either or both having linear projections on the forming surface, with the first die moving toward the second die in such a way that said linear projections cut into the region which has undergone bending or bending-unbending deformation during press-forming, thereby forming the linear concave portions meeting the above-mentioned requirements.